The present invention relates to a method for the degradation of organic materials in a stream of an aqueous medium, such as trade waste, and a multiple vessel reactor system which includes a fluidized bed reactor and a contact reactor, for carrying out this process. The invention further resides in a method for establishing a population of anaerobic bacteria in a multiple vessel reactor system which includes a fluidized bed reactor and a contact reactor.
It is well known that bacterial degradation can remove organic materials from sewage, wastewater and other aqueous media. In an effort to improve the efficiency of such processes fluidized bed techniques have been used to culture aerobic bacteria. The aqueous medium to be treated is pumped upwardly into the bed to fluidized the media in the bed. The aerobic bacteria are grown in the bed and the treated water removed at the top of the column with some of the bed particles. These bed particles must then be cleaned to remove excess biomass generated by the growth of the bacteria and the cleaned particles returned to the column. The large amount of biomass generated in such a fluidized bed is a significant disadvantage in such a system as is the need to provide oxygen for the bacteria.
It has been proposed in U.S. Pat. No. 4,182,675 to remove biochemical oxygen demand (BOD) from waste water by forming a fluidized bed of microorganisms attached to a solid particulate carrier, continuously passing waste water to be treated through said fluidized bed, retaining the waste water in the fluidized bed for a sufficient period of time while controlling other necessary parameters and while maintaining the bed in an anaerobic condition to biologically convert substantially all of the biochemical oxygen demand to be removed from the waste water to methane gas, carbon dioxide and cellulose material. It is also disclosed that nitrified effluent may be added to the waste water and the mixture biologically converted to methane gas, carbon dioxide, nitrogen gas and cellulose material.
The process disclosed in U.S. Pat. No. 4,182,675 suffers from a number of difficulties from a practical point of view. The first of these is that it is difficult to get a population of anaerobic bacteria to remain adhered to the particulate carrier in the bed. Some time after start up there is a tendency for the bacterial layer on the substrate to slough off due to an apparent lack of any inherent capacity of the anaerobic bacteria to adhere to the particulate carrier. This loss of biomass substantially destroys the efficiency of the process. A further difficulty is that the process described does not work efficiently with trade wastes having high BOD concentration, i.e. BOD concentrations of more than 1,500 mg/l, typically more than 2,000 mg/l. In the process described if high BOD wastes are passed through fluidized beds of reasonable size slowly enough to provide residence times sufficient to bring about a substantial reduction of BOD then the bed will not remain fluidized. This latter problem is sought to be overcome in the abovementioned specification by the use of plurality of fluidized beds in series. The present invention provides an alternative mode of overcoming this problem without the excessive cost of providing a plurality of fluidized beds in series.
A further difficulty with the process described in U.S. Pat. No. 4,182,675 is that it is not well adapted to handle changes in the BOD of the aqueous medium. In the prior art arrangement a substantial change in the BOD of the aqueous medium will lead to the degradation of the treated effluent due to the inability of the organisms in the bed to handle the increased BOD. The present invention seeks, by the use of two interconnected reaction tanks, to provide a system which can readily handle the rapid and substantial changes in BOD which occur in the practical handling of trade wastes.